In the prior art, it is a common practice to apply plating, typically nickel electroplating to the surface of rare earth magnets, typically Nd—Fe—B permanent magnets, for the purpose of imparting corrosion resistance thereto. While the plating bath is used over a long term, rare earth metal ions are leached out of the rare earth magnets, i.e., work to be plated and gradually accumulate in the plating bath, adversely affecting plating quality.
In particular, the nickel electroplating bath is sensitive to impurities. Even traces of impurities, if introduced, can incur detrimental phenomena such as adhesion failure, covering power shortage, and hard brittle coatings. Impurities are composed of fine solids suspended in the plating bath or settled on the plating tank bottom and soluble impurities dissolved in the plating bath. Fine solid impurities may be removed by a physical removal method, typically filtration. Of the soluble impurities dissolved in the plating bath, some transition metals may be removed by effecting dummy electrolysis with a low current flow for allowing impurity metal ions to deposit on the cathode. Rare earth metal ions, however, are quite difficult to remove.
For removal of rare earth metal ions such as Nd and Dy in the nickel electroplating bath as impurities, Patent Document 1 proposes a method involving adding less than equivalent of an extractant to the nickel plating bath, mixing and agitating until the extractant is associated with rare earth metal ions to form an associated gel, separating and removing the associated gel from the plating bath, and recovering the plating bath for reuse.
This method, however, has many problems. If more than equivalent of the extractant is added, the extractant does not gel. Then separation is difficult. Complete removal of rare earth metal impurities such as Nd and Dy is impossible. This indicates that expensive rare earth metals such as Nd and Dy are discarded, and the extractant is also discarded. There is left an industrial waste in gel form, which may be accepted by few waste disposal facilities and hardly treated in an industrially acceptable manner. The operation of withdrawing the gel waste poses a burden to the worker.